Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable stimulator.
As shown in FIGS. 1A and 1B, a SCS system typically includes an Implantable Pulse Generator (IPG) 100, which includes a biocompatible device case 30 formed of a conductive material such as titanium for example, or formed of a non-conductive ceramic. The case 30 typically holds the circuitry and battery 26 necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 100 includes one or more electrode arrays (two such arrays 102 and 104 are shown), each containing several electrodes 106. The electrodes 106 are carried on a flexible body 108, which also houses the individual electrode leads 112 and 114 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on array 102, labeled E1-E8, and eight electrodes on array 104, labeled E9-E16, although the number of arrays and electrodes is application specific and therefore can vary. The arrays 102, 104 couple to the IPG 100 using lead connectors 38a and 38b, which are fixed in a non-conductive header material 36, which can comprise epoxy for example.
FIGS. 2A and 2B show circuitry within the IPG 100 for distributing a therapeutic current, Iout=Iin, between the various electrodes. This current is usually provided as pulses. Shown are a single constant current source 60 and a single constant current sink 60′. Because the current source 60 is formed from P-channel transistors, and because its analog output current, Iout, is set by digital signals (Amp<1:M>), the current source 60 is referred to as a P Digital-to-Analog converter, or “PDAC” 60. Similarly, because the current sink 60′ is formed from N-channel transistors, and because its analog input current, Iin, is set by digital signals (Amp<1:M>′), the current sink 60 is referred to as a N Digital-to-Analog converter, or “NDAC” 60′. (Note that prime symbols are used in conjunction with the sink circuitry).
As just mentioned, the current output from, or input to, the PDAC 60 and NDAC 60′ are set by digital amplitude signals Amp and Amp′ respectively. There may be M of such digital signals, which ultimately issue from some type of control circuitry 160 in the IPG 100, such as a microcontroller. It is typical that the PDAC 60 and NDAC 60′ are programmed by Amp and Amp′ to source and sink the same current magnitude, i.e., Iout=Iin. In this way, current injected into the patient's tissue, R (FIG. 2B), from one electrode is drawn back into the IPG 100, and thus surplus charge will not accumulate in the patient.
As is well known, and discussed further in U.S. Patent Application Publication 2007/0038250, which is incorporated herein by reference in its entirety, PDAC 60 and NDAC 60′ comprise current mirrors which amplify a reference current, Iref, to produce the desired source and sunk currents, Iout and Iin, in accordance with the digital signals Amp and Amp′. Each signal Amp<x> and Amp<x>′ controls a switch 61 to cause 2x-1 current mirror transistors 62 and 62′ to be placed in parallel to contribute to the current. This allows the produced currents, Iout and Iin, to be produced as a scalar k of the reference current, i.e., Iout=Iin=kIref. For example, to produce an output current, Iout, of 11Iref, Amp can be set to <00001011>, which places 1+2+8=11 current mirror transistors 62 in parallel. However, this means of digitally setting the output and input currents is merely one example, and other means of setting these currents can also be used, such as are disclosed in the '250 Publication. Ultimately, current flows through the PDAC 60, the tissue R, and the NDAC 60′ by virtue of a compliance voltage (V+) coupled to the PDAC 60, and a reference potential (ground; GND) coupled to the NDAC 60′.
Switch matrices 50 and 50′ allow the current sourced and sunk by PDAC 60 and NDAC 60′ to be distributed to any of the electrodes E1-EN. For example, in FIG. 2B, electrode E1 has been selected to receive the sourced current Iout, while electrode E2 has been selected to receive the sunk current, Iin, thus allowing current to flow through the tissue R between these two electrodes. Selection of the electrodes occurs at switching matrices 50 and 50, and in this example, there are N switches S1-SN and S1′-SN′ in each matrix 50 and 50′ to allow distribution of the currents to each of the N electrodes, E1-EN. Selection of the switches occurs in accordance with switching control signals Switch<1:N> and Switch′<1:N>, which again can be issued by the control circuitry 160. Thus, to select electrodes E1 and E2 as shown in FIG. 2B, switch S1 has been turned on by switch control signal Switch<1>, while switch S2′ has been turned on by switch control signal Switch<2>′. Which of the electrodes are chosen, as well of the amplitude, frequency and duration of the pulses occurring at those electrodes, will be dictated by the patient or clinician, usually based on experimentation as to which settings are most effective, for example, to alleviate the patient's pain or other symptoms.
The inventor has noticed that the current distribution architecture of FIGS. 2A and 2B may not function properly if the IPG 100 experiences certain types of failures. FIG. 3A illustrates the IPG 100 functioning properly, passing 5 mA out electrode E1, through the tissue R, and back into electrode E2. FIGS. 3B and 3C show various failures that affect this desired current flow. In FIG. 3B, there is an open circuit 63 in the path going to electrode E1. This failure could occur anywhere along the path from the PDAC 60 to the electrode E1, including inside the case 30, in the internal connections between the IPG 100 and the lead connectors 38a and 38b, in the lead connectors 38a and 38b, in the leads 112 or 114 themselves, or where the leads 112 or 114 connect to the ring electrodes 106 on the arrays 102 or 104. For example, the lead 112 leading to electrode E1 could have been damaged when it was implanted in the patient, or that lead might be making a poor connection to the contact in its lead connector 38a or 38b. When this failure condition occurs, no current will flow through switch S1 by virtue of the open circuit 63. Likewise, because electrode E2 is isolated from the compliance voltage V+ ultimately used to drive the current, the current through S2′ will also equal zero. Thus, no current flows, despite the programming of the PDAC 60 and NDAC 60′.
In FIG. 3C, there is a short circuit 64 between electrode E1 and ground (GND). This failure can again occur anywhere along the path from the PDAC 60 to the electrode E1. Assuming the short 64 is of significantly lower resistance than the tissue R, the majority of current output from the PDAC 60 (5 mA) will flow though the short 64 to ground. As a result, no current (or negligible current) would flow through the tissue R, and E2 is effectively coupled to ground via the short 64. Because the NDAC 60′ is referenced to ground, no potential exists to drive a current at electrode E2, and thus no current (or negligible current) will flow through switch S2′.
FIGS. 4A-4C are analogous to FIGS. 3A-3C, but show more-complicated examples in which two electrodes (E1 and E2) are chosen to receive the sourced current, Iout=5 mA, while electrode E3 receives the entirety of this current, Iin=5 mA. Splitting either the sourced or sunk current between two or more electrodes can be therapeutically useful for a particular patient. Alternatively, it can be useful to at least temporarily split the sourced or sunk current in this fashion while experimentally “steering” current from one electrode to another to try to find a good therapeutic result for the patient. Current steering is discussed further in U.S. Pat. No. 7,890,182. As shown in FIG. 4A, the source current Iout=5 mA is shared between the selected electrodes E1 and E2, with the result that about half of this current would pass through each of switches S1 and S2, or about 2.5 mA. (The actual amount carried through the switches would depend on the resistive network R of the tissue between the selected electrodes). These currents rejoin at electrode E3, which sinks the entire 5 mA of current.
FIG. 4B shows an open circuit 63 in the path leading to E2. In this circumstance no current would flow through switch S2, and instead the entirety of the source current (5 mA) flows through switch S1 and electrode E1, and through electrode E3 and switch S3.
FIG. 4C shows a short circuit 64 in the path leading to electrode E1. In this circumstance, and assuming the short 64 is of low resistance, the entirety of the sourced current (5 mA) flows through switch S1 through the short 64, and no current (or negligible current) flows through S2 and electrode E2. As with FIG. 3C, effective grounding of electrode E3 prevents a significant current from flowing through electrode E3 and switch S3′.
The inventor finds the failures conditions of FIGS. 3B, 3C, 4B, and 4C regrettable, because in each case the selected electrodes are not receiving the amount of current desired, which ultimately affects patient therapy and potentially also impacts patient safety. A better solution for monitoring these and other failure conditions is therefore warranted, and is provided by this disclosure.